Better way of marking a location for an object to copy
Suggestion.  Try a more casual approach if that is acceptable.


I believe no part of the wrist bone should touch the visible gun. The wrist  placement need not be in the mesh of the gun.   The placement could be near the gun.  See the abstract control note below. The wrist bone maybe able to be specified head to tail for placement.  
You might introduce a gun contact bone non deforming if that suits you.
Consider a [track to] constraint. To control wrist bone or connected bone.
You may need to worry about cyclic dependencies.  Whether real or Blender defect.  You may need to have non rendered abstract effector to control both the placement of gun and the hands.The abstract effector could be a simple armature with bones for the left and right wrist placement and location and rotation of gun.   This may eliminate cyclic dependencies if they arise.  
Currently your question does not show the hierarchical relationships or parent child relationships.
The placement of fingers tips can just be curled.